This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The two great classes of vertebrates capable of sustained high oxygen consumptions, the birds and mammals, share many similarities of physiology but have radically different lungs. There is evidence that the avian pulmonary circulation is very different from that in mammals in that the avian pulmonary capillaries appeared to be essentially rigid. The reason for this are not understood but probably have to do ultrastructure of the tissues surrounding the capillaries, and the facilities of the NCMIR with its high voltage electron microscope tomography are well suited to elucidating this problem.